The present invention relates to an initial hole machining device for a wire-cut electric discharge machine.
In wire-cut electric discharge machining, a wire electrode is first inserted through an initial hole bored through a workpiece before the electric discharge machining process is started. The initial hole is conventionally formed by using a drill, for example. With the use of a drill, however, it is difficult to make an initial hole, particularly a small-diameter hole, in a workpiece which has been heat-treated to have high hardness. Conventionally, therefore, an initial hole machining device utilizing the electric discharge machining power supply of a wire-cut electric discharge machine is used for boring an initial hole by elecrical discharge machining. This type of initial hole machining device typically includes a pipe electrode for initial hole machining which is connected to the electric discharge machining power supply of the wire-cut electric discharge machine by a lead wire. The pipe electrode is arranged in a position horizontally offset from the upper wire guide of the electric discharge machine.
In an initial hole machining process, after a table carrying a workpiece thereon is moved horizontally to set the workpiece in a desired initial hole-machining position, an electrical discharge is produced between the workpiece and the pipe electrode, simultaneously supplying a machining liquid through an initial hole-machining nozzle which is separate from an electric discharge machining nozzle of the upper wire guide, thereby making an initial hole. During a normal electric discharge machining process, on the other hand, the table is moved horizontally to cause the workpiece to move relative to the wire electrode in accordance with a predetermined machining locus while the machining liquid is supplied through the nozzle of the upper wire guide and an electrical discharge is produced between the workpiece and the wire electrode, thereby machining the workpiece.
In such a conventional initial hole machining device in which the pipe electrode is located in a position offset from the wire electrode, the initial hole machinable region is shifted from the electric discharge machinable region by the offset amount. Accordingly, there is a dead zone where only one of the discharge machining process and the initial hole machining process is feasible but the other cannot be carried out. Moreover, to perform the initial hole machining, either the position command data for table position control representing a position to be machined or the machine coordinate system set in the electric discharge machine must be corrected in accordance with the offset amount. In addition, the lead wire connecting between the pipe electrode and the electric discharge machining power supply causes an increase of the inductance of the initial hole machining power supply system, thus deteriorating the characteristics of the electric discharge machining power supply.